1. Field of the Invention
The present invention relates to an electrophotographic image-forming apparatus, and more particularly to a configuration of a toner-supplying roller and a developing roller that receives toner from the toner-supplying roller.
2. Description of the Related Art
A developer (referred to as toner hereinafter) for a conventional electrophotographic image-forming apparatus takes the form of hard, powder particles that are difficult to melt together in a normal storage temperature range. One such toner is manufactured by pulverization and has a particle diameter of about 10 μm. This pulverized toner has a glass transition point Tg higher than 68° C. Thus, pulverized toner is used for its low cost. As the name implies, pulverized toner is manufactured by mechanically pulverizing the toner material. Therefore, the diameter and composition of the toner particles have large variations but could be used without problem when printing is performed at conventional speeds. Another problem is that recent demand for increased printing speed in electrophotography accelerates wear of developing rollers. In order to solve this problem, the amount of nip formed between a developing roller and a toner-supplying roller is increased, thereby controlling the ratio of the circumferential speed of the toner-supplying roller to that of the developing roller to be lower than a predetermined value.
If a conventional electrophotographic image-forming apparatus uses the conventional pulverized toner and operates to print at a higher speed than conventional, a fixing unit requires a larger electric power to fuse a hard-to-melt toner. Greater power consumption generates more heat, requiring efficient heat dissipation. This would result in a larger overall apparatus size. However, there have been strong demands in the field of recent image forming apparatus towards higher printing speed, less power consumption, and miniaturization of apparatus. Conventional image-forming apparatus may be increased in printing speed at the expense of greater power consumption, but are difficult to address the problem of saving electric power and miniaturizing the image-forming apparatus.
For example, a conventional image-forming apparatus requires a developing roller to rotate at a circumferential speed in the range of 70 to 150 mm/sec. Recently, a developing roller is required to rotate at a speed higher than 150 mm/sec. In addition, because the conventional hard-to-melt toner is an obstacle to saving electric power and miniaturizing the image forming apparatus, a low melting point toner has come into use for less power consumption in the fixing processing. A lower melting point toner has either a glass transition point lower than 67° C., or a softening point Ts lower than 80° C. and a fluid point Tfb lower than 120° C. Tfb is a temperature at which the flowing of the toner begins. However, if such a low melting point toner is used with a developing roller rotating at a circumferential speed higher than 150 mm/sec, the toner melts due to the heat created by the friction between the developing roller and the toner-supplying roller and is deposited on the developing roller.